Friction pressure welding has been conventionally known as an art for bonding two members of different shapes to form a bonded product. For example, JP5-84614A discloses that a first preform and a second preform, which are provided with tooth profiles formed by precision forging, are bonded by friction pressure welding, so that a composite gear having a tooth profile part is formed.
Laser welding has been known as another art for bonding two members of different shapes to form a bonded product. For example, JP2006-509172T discloses that a ring gear requiring a high strength and a differential case are laser-welded, so that a differential case having a tooth profile part is formed.
In the art disclosed in JP5-84614A, i.e., in the art for bonding by frictionally pressure-welding the first preform and the second preform, which are provided with the tooth profiles formed by precision forging, one of the first preform and the second preform is rotated at a high speed and pressure-welded to the other, so that an end of the first preform and an end of the second preform are fused by the frictional heat to thereby achieve bonding. Thus, in the bonded product that has been bonded by the friction pressure welding, the first preform and the second preform may be bonded with shaft centers thereof being misaligned, or an axial length of the bonded product may vary depending on a fusing condition of the end of the first preform and the end of the second preform. In this case, there is a problem in that a required precision cannot be obtained in the bonded product that has been bonded by the friction pressure welding. In order to accomplish the required precision in the bonded product that has been bonded by the friction pressure welding, the bonded product that has been bonded by the friction pressure welding has to be subjected to a machining process, which significantly increases a process cost.
On the other hand, in the art disclosed in JP2006-509172T, as particularly shown in the section 0017 and FIG. 1B of this document, a first differential case as the first preform and a second differential case having a ring gear as the second preform are bonded by laser welding, while the first and second differential cases are being respectively fixed, so that the differential case as a bonded product is formed. When the first differential case and the second differential case are welded while they are being fixed, if a shaft center of the first differential case and a shaft center of the second differential case are adjusted to be aligned during the fixing step, misalignment of the shaft centers does not occur whereby a required precision can be achieved in a resulting differential case formed by bonding the first and second differential cases.
However, since a ring gear requires a high strength, the ring gear is often made of high-carbon steel containing a larger amount of C (carbon) and quenched. In this case, when the second differential case made of high-carbon steel is subjected to fusion welding such as laser welding, there is a possibility that a bonded portion of high-carbon steel, which has been fused, hardens and cracks. Namely, a problem may occur in that a strength of the bonded portion where the first differential case and the second differential case are bonded is degraded.